As means for printing the surfaces of various formed articles, such as plastic caps, containers, plastic films and various other plastic formed articles on an industrial scale, there have been widely used gravure printing and flexography. In recent years, there has also been used a printing means based on an ink-jet system.
Printing means based on the ink-jet system consists of flying tiny ink droplets from a nozzle head so as to be deposited and fixed onto a predetermined recording medium to thereby print an image based on the printing data that are input. This printing means has no need of making a plate, enables the designs to be easily changed, and is suited for printing, specifically, a variety of kinds of products of small lots providing, further, an advantage of inexpensive running costs.
In view of the above advantage, a patent document 1, for example, proposes an art of printing images on the top panel of plastic caps relying on the ink-jet system.
The inks for ink-jet printing that have been known can be grouped into those of the ultraviolet ray-curing type and those of the heat-curing solvent type accompanied, however, by their own advantages and disadvantages.
For instance, the ultraviolet ray-curing ink is polymerized and cured upon the irradiation with an ultraviolet ray and, therefore, has such an advantage that it needs no heating and can be cured in short periods of time. However, the ultraviolet ray-curing ink has such defects that it is very expensive and, besides, forms low-molecular components stemming from a starting agent and the like agents as the polymerization/curing takes place, and generates offensive odor.
On the other hand, the heat-curing solvent type ink is inexpensive and is free from generating offensive odor, but requires the step of curing for being polymerized and cured. Further, if it is attempted to cure the ink in short periods of time, the ink must be heated at a temperature very higher than a melting point of the plastic material. Therefore, the plastic formed articles are subject to be thermally deformed if it is attempted to print images on them using the heat-curing solvent type ink.
It has, therefore, been desired to provide a heat-curing solvent type ink for ink-jet printing that polymerizes and cures upon being heated at a low temperature for a short period of time and that, further, closely adheres onto the formed articles.
For instance, a patent document 2 discloses in its Example 9 an art of forming a polyurethane resin layer on the upper surface of a top panel (outer surface of a top plate) of a plastic cap by using a coating solution that contains polyesterpolyol and polyisocyanate. To form the polyurethane resin layer that firmly adheres onto the cap, however, the coating solution must be heated at a temperature close to 130° C. for several minutes often causing a problem of deformation in the cap. In this case, deformation in the cap can be reliably avoided by shortening the time of heating permitting, however, the coating solution to be blurred. Therefore, the coating solution cannot be used as the ink.